An invention that should fit into the current events might be player positioned and built barricades. These would temporarily stop any migrations from the enemy until they remove the blockade. The barricades would be on a timer but could be taken down early by attacking it as you would any other barricade.

It would take time, manpower, and lumber to set up such a defense and all could get involved in construction.
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I think barricades that could be erected by either side and either stormed or shored up would be amazing.

Mazrian